This invention relates to memory cells and more particularly relates to non-volatile memory cells.
Non-volatile memory cells maintain their contents without the need for an external power supply. In comparison, SRAM, DRAM or other memory technologies lose their contents when the power is switched off. An internal battery is sometimes used to mimic non-volatile memory with SRAM or DRAM. However, an internal battery installation is expensive and cannot guarantee proper operation over long periods of time. It is highly desirable to store certain data, such as boot-up code, chip ID, chip self-repair information, etc., in a non-volatile memory.
Improvements in semiconductor technology have increased the performance of integrated circuits while reducing device dimensions. Unfortunately, conventional techniques for designing planar non-volatile memory cells implemented in standard digital CMOS processes as well as three-dimensional (double-stacked polysilicon) cells implemented using specialized processes have not been able to address the negative effects of leakage currents. Leakage currents give rise to data retention and sense margin issues in a nonvolatile memory as the stored charge decreases due to such undesired leakage currents. Such leakage currents are especially significant in advanced technologies where thin gate oxides are used for the gate dielectric in transistors.
In non-volatile memories, the storage node of a memory cell is prone to leakage currents. Sensing at the storage node of a memory cell is typically performed by way of single-ended sensing, requiring larger voltage margins in order to compensate for loss of charge due to leakage currents. Using current techniques, if an erased cell loses a significant amount of charge, the cell's ability to sense the correct voltage value at a storage node will fail.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.